1. Field of the Invention
The present invention relates to centrifugal decanters of the pendulum type and more particularly to those in which the rotor and the frame containing the swivel joint drive mechanism are separated from each other by a horizontal protecting slab.
2. History of the Art
In machines of this type, the vertical shaft which connects the drive motor to the rotor and which passes through the protecting slab is known to be mounted in an oscillating sleeve or bearing provided with an annular bearing surface of semispherical profile which rests on a fixed dish of corresponding concave profile. This conventional arrangement allows self-alignment of the rotor and thus avoids the effects of the inevitable unbalance of said rotor during its rotation at high speed. Of course, shock absorbers are provided, disposed between a flange fast with the bearing and the outer edge of the dish, the purpose of these absorbers being to dampen the oscillations of the pendular assembly at normal operational speeds.
However, it may happen, particularly in the event of an earthquake, that the amplitude of the oscillation of the vertical shaft increases suddenly due to the appearance of considerable vibrations or a marked unbalance of the rotor; in such a case, the shaft risks coming into contact with a fixed part, for example the edge of the opening made in the slab for passage thereof, and it goes without saying that such a shock may prove to be highly dangerous, particularly when the liquid being treated is of radio-active or very strongly corrosive nature.
With a view to setting aside this risk, it has been proposed to provide the decanters with the safety system shown in FIG. 1 of the accompanying drawings. In this figure, reference 1 designates the vertical shaft which connects the upper motor 2 to the rotor or rotating decantation vessel 3, said shaft being engaged with clearance through an opening 4a made in the protecting slab 4; below its pivoting part, the bearing 5 which serves to guide the shaft 1 receives a control piece 6 provided at its base with a semi-spherical bead housed, with a clearance which is as small as possible, inside the central opening of a bell-shaped member 7, the free edge of the latter being engaged in an annular depression 4b made in the upper face of the slab 4 concentrically to the opening 4a.
Depression 4b is filled with oil and between the free edge of the bell 7 and the bottom of said depression are inserted balls or other sliding elements, with the result that, when, under the effect of an exaggerated pendular movement of the shaft 1, consecutive in particular to an earthquake, the piece 6 moves said bell horizontally, the outer edge of the latter bears against the corresponding edge of the depression 4b, limiting the amplitude of he angular movement to a value which avoids any contact between the shaft 1 or rotor 3 and a fixed part of the machine. The assembly 6-7 consequently acts as a safety means which completes the damping effected by the conventional shock absorbers 8 and the lateral spring 9 which may be associated therewith.
Despite appearances, this solution is not entirely satisfactory. In fact, it eliminates the aptitude of the pendular assembly to ensure self-alignment of the rotor beyond a certain unbalanced mass; it will be readily understood that, as soon as the bell-shaped member 7 comes into contact with the edge of the depression 4b, the dynamic operation of the rotor 3 passes suddenly from pendular mode on swivel joint to the mode of a shaft fitted between rigid roller bearings, so that, taking into account the importance of the mass in rotation and of its overhang, the dynamic forces generated at that moment are such that they may cause permanent deformation of the shaft 1 whose rotor 3 may then come into contact with the adjacent fixed parts. The risks which it was hoped to eliminate thus remain.